1. Field of the Invention
The present invention relates to anti-counterfeit technology of printed material.
2. Description of the Related Art
Recent improvement in the image quality of copiers has enabled the production of color copies with small degradation in image quality compared to the original document. Consequently, documents such as monetary instruments, identification documents, or important documents have been forged by using such copiers. Therefore, the need for technology to counteract forgery of printed documents has increased.
To serve this need, technology for hiding a latent image, such as a VOID pattern, in an image of a printed document by utilizing the coarseness and fineness of the pattern of the image and for making the latent image appear when the printed document is color copied has been developed. (For example, refer to U.S. Pat. No. 4,227,719.) This technology cannot control the copying process, for instance, by inhibiting the production of copies. The technology, however, has a deterrent effect on forgery by making a VOID pattern appear in the copied image, thus indicating that the copied image is not an original. In the following, such an image of a printed document including a latent image that appears when the printed document is copied is referred to as a ‘patterned image.’
Even when a legitimate user (e.g., the owner of the original image) copies the original document including a patterned image, a latent image appears, and so the appearance of the copied image differs from the appearance of the original. This can be an advantage from the aspect of anti-counterfeiting of printed materials. This, however, is also a disadvantage in that the copying of the printed material cannot be precisely controlled. For recent copyright protection and document control systems, a system for granting legitimate users with various permissions for using certain images or documents by verifying the user's ownership or payment of a registration fee and for rejecting non-legitimate users is required. Unfortunately, the known technology based on patterned images does not meet this requirement.
At the same time, digital watermarks have recently gained attention for protecting copyrights. Digital watermarking is a technology for hiding a watermark in digital data such as images, sounds, or documents in a way unperceivable to humans. For example, there are various methods for known digital watermark technologies applied to multi-valued images utilizing the redundancy of the multi-valued pixel intensity. Another example of a method for known digital watermark technology is a method for imbedding a digital watermark in the spatial domain, known as a patchwork (for instance, refer to U.S. Pat. No. 5,636,292). Another known method for imbedding a digital watermark in the frequency domain is discrete cosine transform.
Binary images such as document images have less redundancy compared to multi-valued images, and, thus, it is difficult to apply digital watermark technology. In spite of this difficulty, there are several known digital watermark methods utilizing the characteristics of document images (for instance, refer to U.S. Pat. No. 5,861,619).